add comments

Author: Pedro Fonseca

src/Renci.SshNet/ServiceFactory.cs

@@ -164,6 +164,8 @@ public ISftpFileReader CreateSftpFileReader(string fileName, ISftpSession sftpSe
             {
                 var fileAttributes = sftpSession.EndLStat(statAsyncResult);
                 fileSize = fileAttributes.Size;
+
+                // calculate maxPendingReads based on remaining size, not total filesize (needed for resume support)
                 maxPendingReads = Math.Min(10, (int) Math.Ceiling((double)(fileSize - (long)offset) / chunkSize) + 1);
             }
             catch (SshException ex)

src/Renci.SshNet/Sftp/SftpFileReader.cs

@@ -63,15 +63,18 @@ public SftpFileReader(byte[] handle, ISftpSession sftpSession, uint chunkSize, i
             _sftpSession = sftpSession;
             _chunkSize = chunkSize;
             _fileSize = fileSize;
-            _offset = offset;
-            _readAheadOffset = offset;
             _semaphore = new SemaphoreLight(maxPendingReads);
             _queue = new Dictionary<int, BufferedRead>(maxPendingReads);
             _readLock = new object();
             _readAheadCompleted = new ManualResetEvent(initialState: false);
             _disposingWaitHandle = new ManualResetEvent(initialState: false);
             _waitHandles = _sftpSession.CreateWaitHandleArray(_disposingWaitHandle, _semaphore.AvailableWaitHandle);
 
+            // When resuming a download (offset > 0), set the initial offset of the remote file to
+            // the same offset as the local output file. Read-ahead also starts at the same offset.
+            _offset = offset;
+            _readAheadOffset = offset;
+
             StartReadAhead();
         }
 

src/Renci.SshNet/SftpClient.cs

@@ -980,6 +980,7 @@ public void UploadFile(Stream input, string path, bool canOverride, Action<ulong
 
             if (input.Position > 0)
             {
+                // if the local stream position is not zero, open the remote file in APPEND mode to resume upload
                 flags = Flags.Write | Flags.Append;
             }
             else if (canOverride)
@@ -1122,6 +1123,7 @@ public IAsyncResult BeginUploadFile(Stream input, string path, bool canOverride,
 
             if (input.Position > 0)
             {
+                // if the local stream position is not zero, open the remote file in APPEND mode to resume upload
                 flags = Flags.Write | Flags.Append;
             }
             else if (canOverride)
@@ -2432,6 +2434,7 @@ private void InternalUploadFile(Stream input, string path, Flags flags, SftpUplo
 
             var handle = _sftpSession.RequestOpen(fullPath, flags);
 
+            // Set the initial offset of the remote file to the same as the local file to allow resuming
             var offset = (ulong)input.Position;
 
             // create buffer of optimal length